1. Field of the Invention
Particularly although not exclusively, the servomotor may be a double acting hydraulic cylinder. Such control units have the advantage that, if all of the hydraulic fluid used in controlling the steering of the vehicle passes through the motor, the motor is also able to function as a pump in the event the external pump should fail, a valve system is employed to directly connect a suction circuit to the reservoir to maintain sufficient hydraulic fluid in the circuit of the control unit and the servomotor (cylinder) to effect the steering function.
2. Prior Art
Control units as used in the prior art have traditionally employed rotatable distributor valves positioned between the control shaft and the hydraulic motor.
An example of such a prior art mechanism is disclosed in the German Pat. Specification No. 1,293,029.
This known prior art arrangement has the disadvantage of being difficult to achieve a seal between the exterior face of the sleeve of the distributor valve and the adjacent internal face of the housing because of the rotation of the sleeve together with the shaft controlling the hydraulic motor. It has been found in practice that the prior art valve distribution system becomes less responsive with increasing use, resulting in requiring greater wheel turning to achieve the same steering effect, with concomitant vehicle occupant safety problems.